Photoelectric conversion device and image generation device

ABSTRACT

A photoelectric conversion device includes a photoelectric conversion unit which includes a phototransistor having a collector region, an emitter region, and a base region to generate an output current according to an intensity of incident light to the phototransistor, and a base potential setting unit which is configured to set up a base potential of the phototransistor so that the output current from the photoelectric conversion unit is equal to a predetermined current value.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims the benefit of priorityof Japanese Patent Application No. 2015-131277, filed on Jun. 30, 2015,the contents of which are incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to a photoelectric conversion device andan image generation device.

2. Description of the Related Art

Conventionally, a photoelectric conversion cell utilizing aphototransistor having a photocurrent amplification function is known asa photoelectric transducer. For example, see Japanese Patent No.5674096. In this photoelectric conversion cell, a process to discharge(reset) electric charge accumulated in the phototransistor thereof isperformed by discharging (resetting) the electric charge accumulated inthe phototransistor upon reading of the photoelectric conversion cell.

However, the photoelectric conversion cell may have a difficulty infully resetting the electric charge accumulated in the phototransistorwithin a predetermined reading time when intense optical energy isreceived, or when an extended accumulation time is required.

SUMMARY OF THE INVENTION

In one aspect, the present disclosure provides a photoelectricconversion device capable of reducing the time needed to reset theelectric charge accumulated in the phototransistor.

In one embodiment, the present disclosure provides a photoelectricconversion device which includes a photoelectric conversion unit whichhas a collector region, an emitter region, and a base region, andincludes a phototransistor to generate an output current according to anintensity of incident light to the phototransistor, and a base potentialsetting unit which is configured to set up a base potential of thephototransistor so that the output current from the photoelectricconversion unit is equal to a predetermined current value.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory and are not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit diagram illustrating a photoelectricconversion device according to a first embodiment.

FIG. 2 is a flowchart for explaining operation of the photoelectricconversion device according to the first embodiment.

FIG. 3 is a diagram illustrating a state of the photoelectric conversiondevice according to the first embodiment when performing an accumulationprocess.

FIG. 4 is a diagram illustrating a state of the photoelectric conversiondevice according to the first embodiment when performing an integratorreset process.

FIG. 5 is a diagram illustrating a state of the photoelectric conversiondevice according to the first embodiment when performing a cell selectprocess.

FIG. 6 is a diagram illustrating a state of the photoelectric conversiondevice according to the first embodiment when performing a cell resetprocess.

FIG. 7 is an equivalent circuit diagram illustrating a photoelectricconversion device according to the related art.

FIG. 8 is a diagram for explaining changes of a base-to-emitter voltagewhen resetting a phototransistor.

FIG. 9A and FIG. 9B are graphs illustrating a relationship between anoutput voltage of an integrator and the elapsed time.

FIG. 10 is an equivalent circuit diagram illustrating a photoelectricconversion device according to a second embodiment.

FIG. 11 is a flowchart for explaining operation of the photoelectricconversion device according to the second embodiment.

FIG. 12 is a diagram illustrating a state of the photoelectricconversion device according to the second embodiment when performing theaccumulation process.

FIG. 13 is a diagram illustrating a state of the photoelectricconversion device according to the second embodiment when performing theintegrator reset process.

FIG. 14 is a diagram illustrating a state of the photoelectricconversion device according to the second embodiment when performing thecell select process.

FIG. 15 is a diagram illustrating a state of the photoelectricconversion device according to the second embodiment when performing thecell reset process.

FIG. 16 is an equivalent circuit diagram illustrating a photoelectricconversion device according to a third embodiment.

FIG. 17 is a flowchart for explaining operation of the photoelectricconversion device according to the third embodiment.

FIG. 18 is a diagram illustrating a state of the photoelectricconversion device according to the third embodiment when performing theaccumulation process.

FIG. 19 is a diagram illustrating a state of the photoelectricconversion device according to the third embodiment when performing theintegrator reset process.

FIG. 20 is a diagram illustrating a state of the photoelectricconversion device according to the third embodiment when performing thecell select process.

FIG. 21 is a diagram illustrating a state of the photoelectricconversion device according to the third embodiment when performing thecell reset process.

FIG. 22 is a diagram illustrating a configuration of an image generationdevice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will be given of embodiments with reference to theaccompanying drawings.

First Embodiment

First, a configuration of a photoelectric conversion device 1 accordingto a first embodiment will be described. FIG. 1 is an equivalent circuitdiagram illustrating the photoelectric conversion device 1 according tothe first embodiment.

As illustrated in FIG. 1, the photoelectric conversion device 1 includesa pixel cell 10, an integrator 20 disposed for the pixel cell 10, anoutput line 30 disposed as a path for reading signal charge, a constantcurrent source 40 disposed to branch OFF from a path of the output line30, and a reset line 50 disposed to connect the pixel cell 10 and anoutput of the integrator 20.

The pixel cell 10 is an example of a photoelectric conversion unit, andthe above-described configuration including the integrator 20, theoutput line 30, the constant current source 40, and the reset line 50 isan example of a base potential setting unit.

The pixel cell 10 includes a voltage source V_(cc), a phototransistor11, a cell select switch 12 (which is disposed as a signal chargereading unit), and a cell reset switch 13.

The phototransistor 11 has a bipolar structure including a collectorregion C, an emitter region E, and a base region B. The phototransistor11 generates an output current according to an intensity of incidentlight to the phototransistor 11.

The collector region C is connected to the voltage source V_(cc).

The emitter region E is connected to the cell select switch 12. When thecell select switch 12 is turned ON, the emitter region E of thephototransistor 11 is connected to the output line 30 via the cellselect switch 12. The integrator 20 is connected to the output line 30,and the signal charge from the emitter region E is transferred to theintegrator 20.

The base region B is connected to the cell reset switch 13. When thecell reset switch 13 is turned ON, the base region B is connected to thereset line 50 via the cell reset switch 13. The output of the integrator20 is connected to the reset line 50, and the output of the integrator20 is supplied to the base region B.

The integrator 20 is disposed to the output line 30 from which aphotocurrent output from the phototransistor 11 is received. Theintegrator 20 is configured to perform an integration process of thephotocurrent output from the phototransistor 11. In the integrationprocess, the electric charge according to the photocurrent isaccumulated within a reading time, the accumulated charge is convertedinto a voltage, and the voltage is output from the integrator 20. Theintegrator 20 includes an operational amplifier 21, a capacitor 22, anintegration capacitor connect switch 23, and an integrator reset switch24.

The integration capacitor connect switch 23 may be a switch configuredto connect the capacitor 22 to the operational amplifier 21 anddisconnect the connector 22 from the operational amplifier 21. Theintegrator reset switch 24 may be a switch configured to discharge thesignal charge accumulated in the capacitor 22 (or configured to performan integrator reset process).

The constant current source 40 is connected via the constant currentsource connect switch 41 to the output line 30 which outputs thephotocurrent from the phototransistor 11.

Next, an example of operation of the photoelectric conversion device 1according to the first embodiment will be described. FIG. 2 is aflowchart for explaining operation of the photoelectric conversiondevice 1 according to the first embodiment. FIGS. 3 to 6 are diagramsillustrating various states of the photoelectric conversion device 1according to the first embodiment when performing various processes.

Upon reception of instructions from a control unit (not illustrated) viacontrol lines 61 and 62, the photoelectric conversion device 1 performsthe processes of steps S11 to S14 as illustrated in FIG. 2.

First, the photoelectric conversion device 1 performs an accumulationprocess to accumulate signal charge in the phototransistor 11.Specifically, the cell select switch 12 is turned OFF and the cell resetswitch 13 is turned OFF (step S11). At this time, in the photoelectricconversion device 1, as illustrated in FIG. 3, the cell select switch 12is in an OFF state and the cell reset switch 13 is in an OFF state. Notethat the integration capacitor connect switch 23, the integrator resetswitch 24, and the constant current source connect switch 41 may be inan ON state or in an OFF state. In the phototransistor 11, uponreception of incident optical energy, the optical energy is convertedinto signal charge, and the signal charge is accumulated in the baseregion B. In this condition, the emitter region E and the base region Bof the phototransistor 11 in the pixel cell 10 are in a floating state.Hence, the signal charge generated according to the optical energy isaccumulated in the base region B, starting from a base potential set upby a cell reset process (which will be described later), and abase-to-collector voltage V_(BC) of the phototransistor 11 is reduced.

Subsequently, the photoelectric conversion device 1 performs anintegrator reset process to discharge the signal charge in the capacitor22 of the integrator 20. Specifically, the integrator reset switch 24 isturned ON and the integration capacitor connect switch 23 is turned ON(step S12). At this time, in the photoelectric conversion device 1, asillustrated in FIG. 4, the cell select switch 12 is in an OFF state, thecell reset switch 13 is in an OFF state, the integration capacitorconnect switch 23 is in an ON state, and the integrator reset switch 24is in an ON state. Note that the constant current source connect switch41 may be in an ON state, or in an OFF state. For example, the constantcurrent source connect switch 41 may be in an OFF state. Thereby, thesignal charge in the capacitor 22 may be discharged.

Subsequently, the photoelectric conversion device 1 performs a cellselect process to discharge the signal charge accumulated in thephototransistor 11. Specifically, the cell select switch 12 is turned ONand the integrator reset switch 24 is turned OFF (step S13). When theconstant current source connect switch 41 is in an OFF state, theconstant current source connect switch 41 is turned ON. At this time, inthe photoelectric conversion device 1, as illustrated in FIG. 5, thecell select switch 12 is in an ON state, the cell reset switch 13 is inan OFF state, the integration capacitor connect switch 23 is in an ONstate, the integrator reset switch 24 is in an OFF state, and theconstant current source connect switch 41 is in an ON state. Thereby,the photocurrent according to the signal charge accumulated in thephototransistor 11 flows into the integrator 20. In the integrator 20,the photocurrent generated in the reading time is accumulated, thephotocurrent is converted into the voltage, and the voltage is output.Note that after an end of the cell select process, the constant currentsource connect switch 41 is turned OFF, and reading of the output of theintegrator 20 is performed. After the reading of the output of theintegrator 20 is performed, the constant current source connect switch41 is turned ON again. The signal output from the integrator 20 isconverted into a digital signal by an AD converter (which will bedescribed later), and the photoelectric conversion device 1 outputs thedigital signal. Note that the constant current source 40 is connected tothe output line 30, and the photocurrent flows into the integrator 20after a constant current i_(B) set up by the constant current source 40is reduced from the photocurrent output from the phototransistor 11.

Subsequently, the photoelectric conversion device 1 performs a cellreset process to discharge the signal charge accumulated in thephototransistor 11. Specifically, the cell reset switch 13 is turned ONand the integration capacitor connect switch 23 is turned OFF (stepS14). At this time, in the photoelectric conversion device 1, asillustrated in FIG. 6, the cell select switch 12 is in an ON state, thecell reset switch 13 is in an ON state, the integration capacitorconnect switch 23 is in an OFF state, and the integrator reset switch 24is in an OFF state. Thereby, a feedback loop is formed by the baseregion B and the emitter region E of the phototransistor 11, the cellselect switch 12, the output line 30, the operational amplifier 21 ofthe integrator 20, the reset line 50, and the cell reset switch 13. Anelectric potential of the output line 30 is controlled to be equal to areference voltage V_(ref), and the base-to-emitter voltage V_(BE) of thephototransistor 11 is controlled to be equal to a value that allows thepassage of the current i_(B) set up by the constant current source 40even if the phototransistor 11 has different characteristics.

Subsequently, after the process of step S14 is completed, the operationof the photoelectric conversion device 1 is returned to step S11 again,and the above-described processes of steps S11 to S14 are repeated.

Next, a photoelectric conversion device of a comparative example inwhich a base region of a phototransistor is set in a floating state willbe described for the purpose of comparison with the photoelectricconversion device 1 according to the first embodiment. FIG. 7 is anequivalent circuit diagram illustrating a photoelectric conversiondevice 9 according to the related art.

As illustrated in FIG. 7, the photoelectric conversion device 9 includesa pixel cell 10, an integrator 20 disposed for the pixel cell 10, and anoutput line 30 as a path for reading signal charge. The pixel cell 10,the integrator 20, and the output line 30 of this photoelectricconversion device 9 are essentially the same as corresponding elementsof the photoelectric conversion device 1 according to the firstembodiment, and a description thereof will be omitted. In thephotoelectric conversion device 9 of the comparative example, the cellselect process is set to the cell select process and the cell resetprocess in combination.

Operation of the photoelectric conversion device 9 of the comparativeexample will be described.

The base region B of the phototransistor 11 is set in a floating stateimmediately after the accumulation process is initiated, acollector-to-base voltage V_(CB) is fixed due to the parasiticcapacitance. The optical energy is received at the phototransistor 11 inthis condition, the signal charge according to the optical energy isaccumulated in the phototransistor 11, and the collector-to-base voltageV_(CB) is reduced. At this time, the base potential increases as theoptical energy increases. In the meantime, the emitter potential is in afloating state, and the signal charge accumulated in the parasiticcapacitance between the base and the emitter is maintained so that thebase-to-emitter voltage V_(BE) indicates a constant value.

When the cell select switch 12 is turned ON, the pixel cell 10 isselected. When the emitter potential is fixed to the reference voltageV_(ref), the signal charge accumulated in the base region B is convertedinto a base current, and the current flows toward the emitter. At thistime, by the bipolar transistor amplification action, the current flowsfrom the collector region C to the emitter region E, and such currentcombined with the base current results in the photocurrent.

Immediately after the pixel cell 10 is selected, the base currentincreases steeply and the base potential decreases greatly. However,with the passage of time, the decrease amount of the base potential isreduced gradually, and the base potential will reach a fixed value. Inthis manner, the decrease amount of the base potential is reduced withthe passage of time, and there may be a case in which the reading timeof the signal charge elapses before the base potential reaches the fixedvalue. In such a case, the base potential before reaching the fixedvalue remains as the potential when the phototransistor 11 is reset, andthe following accumulation process is performed. Hence, the amount ofthe signal charge accumulated by that accumulation process is affectedby the state of the phototransistor before that accumulation process isinitiated, and the amount of the signal charge may deviate from anaccurate amount of the signal charge generated according to the opticalenergy (a reset error). Note that the base potential when thephototransistor 11 is reset varies depending on the amount of opticalenergy received by the phototransistor 11.

Specifically, as illustrated in FIG. 8, the base-to-emitter voltageV_(BE) at a reading end time (“t” indicated in FIG. 8) increases as theamount of optical energy received by the phototransistor 11 increases.FIG. 8 illustrates changes of the base-to-emitter voltage V_(BE) of thephototransistor 11 when the phototransistor 11 is reset. In FIG. 8, thehorizontal axis indicates the elapsed time, and the vertical axisindicates the base-to-emitter voltage V_(BE). Further, in FIG. 8, thesolid line expresses a case in which a large amount of optical energy isreceived by the phototransistor 11, and the long dashed line expresses acase in which a small amount of optical energy is received by thephototransistor 11.

In the photoelectric conversion device 1 according to this embodiment,however, the reset process is performed to establish the base-to-emittervoltage V_(BE) of the phototransistor 11 that allows the passage of theconstant current in which is set up by the constant current source 40.

Hence, in the photoelectric conversion device 1 according to thisembodiment, the base potential is quickly set earlier than a time forthe base potential to reach the potential of the base region B in afloating state which is completely reset when the potential of theemitter region E is fixed. Consequently, reset errors may be reducedeven if the amount of optical energy received by the phototransistor 11varies and the amount of the signal charge accumulated in thephototransistor 11 varies.

In the photoelectric conversion device 1 according to this embodiment,it is possible to set up bias conditions between the base and theemitter of the phototransistor 11. Hence, by setting up the biasconditions beforehand to perform the reset process at high speed, thetime (reset time) needed to reset the electric charge accumulated in thephototransistor may be reduced and the reading time may be reduced.

When the photoelectric conversion device 1 includes a plurality of pixelcells 10, the phototransistor 11 is disposed in each of the pixel cells10. However, such phototransistors 11 may have variations in theircharacteristics. When the phototransistors 11 have variations in theircharacteristics, the following problem may take place. Each of thephototransistors 11 is reset by fixing the base potential to the resetpotential. In such a case, if the reset potential is fixed for all thepixel cells 10, the output current when the base-to-emitter voltageV_(BE) determined according to the reset condition is supplied may havevariations due to the variations in the characteristics of thephototransistors 11.

However, in the photoelectric conversion device 1 according to thisembodiment, even when the phototransistors 11 have variations in theircharacteristics, the reset process is performed so that the identicalemitter current is output from each of the phototransistors 11.Consequently, the variations in the characteristics of thephototransistors 11 may be prevented.

Next, a relationship between the output voltage of the integrator 20 andthe elapsed time will be described. FIGS. 9A and 9B illustrate arelationship between the output voltage of the integrator and theelapsed time.

Specifically, FIG. 9A illustrates a relationship between the outputvoltage of the integrator 20 in the photoelectric conversion device 1 ofthe first embodiment and the elapsed time. FIG. 9B illustrates arelationship between the output voltage of the integrator 20 in thephotoelectric conversion device 9 of the comparative example and theelapsed time. The vertical axis indicates the output voltage V_(out) ofthe integrator 20, and the horizontal axis indicates the elapsed time.

In the upper portion of each of FIGS. 9A and 9B, operating states of thecell select switch 12 and the integrator reset switch 24 areillustrated. In the lower portion of each of FIGS. 9A and 9B, therelationship between the output voltage of the integrator 20 and theelapsed time.

As illustrated in FIG. 9A, in the case of the photoelectric conversiondevice 1 of the first embodiment, when the integrator reset switch 24 isturned ON (time t1), the integrator reset process is performed and theoutput voltage V_(out) is set to the reference voltage V_(ref).

Subsequently, after the integrator reset switch 24 is turned OFF, thecell select switch 12 is turned ON (time t2). At this time, the cellselect process is performed and the output voltage V_(out) changesaccording to the amount of the signal charge accumulated in thephototransistor 11. Specifically, when no signal charge is accumulatedin the phototransistor 11 (a dark condition), the current flows from theintegrator 20 to the constant current source 40, and the output voltageV_(out) increases with the passage of time as indicated by the solidline in FIG. 9A. When a small amount of the signal charge is accumulatedin the phototransistor 11, the photocurrent flows from the pixel cell 10to the integrator 20 initially, and the output voltage V_(out) decreasesinitially. However, with the passage of time, the current flows from theintegrator 20 to the constant current source 40, and the output voltageV_(out) increases gradually as indicated by the long dashed line in FIG.9A. Further, when a large amount of the signal charge is accumulated inthe phototransistor 11, the photocurrent flows from the pixel cell 10 tothe integrator 20, and the output voltage V_(out) decreases with thepassage of time as indicated by the one-dotted chain line in FIG. 9A.

Subsequently, the cell select switch 12 is turned OFF and the cellselect process is terminated (time t3). At this time, the output voltageV_(out) of the integrator 20 is maintained at a constant value.

In this manner, in the case of the photoelectric conversion device 1according to the first embodiment, when no signal charge is accumulatedin the phototransistor 11 (the dark condition), the current flows fromthe integrator 20 to the constant current source 40, and the outputvoltage V_(out) increases with the passage of time. The output voltageV_(out) at this time is higher than the reference voltage V_(ref) by avoltage level represented by (the current of the constant current source40)×(the reading time)/(the capacity of the capacitor 22). Accordingly,the dynamic range becomes wide and the S/N ratio becomes great.

On the other hand, in the case of the photoelectric conversion device 9of the comparative example, the photocurrent of the pixel cell 10 flowsinto the integrator 20 regardless of the amount of the signal chargeaccumulated in the phototransistor 11, and the output voltage V_(out) ofthe integrator 20 is always lower than the reference voltage V_(ref).Accordingly, the dynamic range becomes narrow and it is difficult toobtain a great S/N ratio.

Specifically, as illustrated in FIG. 9B, in the case of thephotoelectric conversion device 9 of the comparative example, when theintegrator reset switch 24 is turned ON (time t1), the integrator resetprocess is performed, and the output voltage V_(out) is set to thereference voltage V_(ref).

Subsequently, after the integrator reset switch 24 is turned OFF, thecell select switch 12 is turned ON (time t2). At this time, the cellselect process is performed. The output voltage V_(out) changesaccording to the amount of the signal charge accumulated in thephototransistor 11. Specifically, when no signal charge is accumulatedin the phototransistor 11 (a dark condition), the current does not flowfrom the phototransistor 11 to the integrator 20 and the output voltageV_(out) remains unchanged as indicated by the solid line in FIG. 9B.When a small amount of the signal charge is accumulated in thephototransistor 11, the current flows from the phototransistor 11 to theintegrator 20, and the output voltage V_(out) decreases as indicated bythe long dashed line FIG. 9B. Further, when a large amount of the signalcharge is accumulated in the phototransistor 11, the amount of thecurrent flowing to the integrator 20 from the phototransistor 11 in thiscase is larger than that in the case in which the small amount of thesignal charge is accumulated in the phototransistor 11, and the outputvoltage V_(out) decreases further as indicated by the one-dotted chainline in FIG. 9B.

Subsequently, the cell select switch 12 is turned OFF (time t3) and thecell select process is terminated. The output voltage V_(out) of theintegrator 20 is maintained at a constant value.

Second Embodiment

A configuration of a photoelectric conversion device 2 according to asecond embodiment will be described. FIG. 10 is an equivalent circuitdiagram illustrating the photoelectric conversion device 2 according tothe second embodiment.

As illustrated in FIG. 10, the photoelectric conversion device 2according to the second embodiment is essentially the same as thephotoelectric conversion device 1 according to the first embodiment,except that the photoelectric conversion device 2 further includes anoperational amplifier (feedback amplifier) 210 configured to perform thecell reset process and disposed to be separate from the operationalamplifier 21 of the integrator 20, and an integrator connect switch 220configured to be switched between an ON state to connect the output line30 to the integrator 20 and an OFF state to disconnect the output line30 from the integrator 20. Other elements of the photoelectricconversion device 2 of the second embodiment are essentially the same ascorresponding elements of the photoelectric conversion device 1 of thefirst embodiment, and a description thereof will be omitted.

As illustrated in FIG. 10, the photoelectric conversion device 2includes the pixel cell 10, the integrator 20, the output line 30, theconstant current source 40, the reset line 50, the feedback amplifier210, and the integrator connect switch 220.

The pixel cell 10 is an example of the photoelectric conversion unit,and the above-described configuration including the output line 30, theconstant current source 40, the reset line 50, the feedback amplifier210, and the integrator connect switch 220 is an example of the basepotential setting unit.

The feedback amplifier 210 is disposed to branch OFF from the path ofthe output line 30, and an output of the feedback amplifier 210 isconnected to the base region B of the phototransistor 11 via the cellreset switch 13.

The integrator connect switch 220 may be a switch disposed on the outputline 30 to connect the pixel cell 10 to the integrator 20 or todisconnect the pixel cell 10 from the integrator 20. The pixel cell 10is connected to the integrator connect switch 220, and when theintegrator connect switch 220 is turned ON, the pixel cell 10 isconnected to the integrator 20 via the integrator connect switch 220.

Next, an example of operation of the photoelectric conversion device 2according to the second embodiment will be described. FIG. 11 is aflowchart for explaining operation of the photoelectric conversiondevice according to the second embodiment.

Upon reception of instructions from a control unit (not illustrated) viathe control lines 61 and 62, the photoelectric conversion device 2performs various processes of steps S21 to S24 as illustrated in FIG.11. FIGS. 12 to 15 are diagrams illustrating various states of thephotoelectric conversion device 2 according to the second embodimentwhen performing the various processes.

First, the photoelectric conversion device 2 performs an accumulationprocess to accumulate signal charge in the phototransistor 11.Specifically, the cell select switch 12 is turned OFF and the cell resetswitch 13 is turned OFF (step S21). At this time, in the photoelectricconversion device 2, as illustrated in FIG. 12, the cell select switch12 is in an OFF state and the cell reset switch 13 is in an OFF state.Note that the integrator reset switch 24 and the integrator connectswitch 220 may be in an ON state or in an OFF state. In thephototransistor 11, upon reception of an incident optical energy, theoptical energy is converted into signal charge. In this condition, theemitter region E and the base region B of the phototransistor 11 in thepixel cell 10 are in a floating state. Hence, the signal chargegenerated according to the optical energy is accumulated in the baseregion B starting from the base potential set up by the cell resetprocess (which will be described later), and the base-to-collectorvoltage V_(BC) of the phototransistor 11 is reduced.

Subsequently, the photoelectric conversion device 2 performs anintegrator reset process to discharge the signal charge in the capacitor22 of the integrator 20. Specifically, the integrator reset switch 24 isturned ON (step S22). At this time, in the photoelectric conversiondevice 2, as illustrated in FIG. 13, the cell select switch 12 is in anOFF state, the cell reset switch 13 is in an OFF state, and theintegrator reset switch 24 is in an ON state. Note that the integratorconnect switch 220 at this time may be in an ON state or in an OFFstate. It is preferable that the integrator connect switch 220 is in anOFF state, in order to reduce the error due to the ON resistance whenthe current flows through the integrator connect switch 220. Byperforming the integrator reset process, the signal charge in thecapacitor 22 may be discharged.

Subsequently, the photoelectric conversion device 2 performs a cellselect process to discharge the signal charge accumulated in thephototransistor 11. Specifically, the cell select switch 12 is turnedON, the integrator reset switch 24 is turned OFF, and the integratorconnect switch 220 is turned ON (step S23). At this time, in thephotoelectric conversion device 2, as illustrated in FIG. 14, the cellselect switch 12 is in an ON state, the cell reset switch 13 is in anOFF state, the integrator reset switch 24 is in an OFF state, and theintegrator connect switch 220 is in an ON state. Hence, the photocurrentaccording to the signal charge accumulated in the phototransistor 11flows into the integrator 20. In the integrator 20, the photocurrentgenerated in the reading time is accumulated and the photocurrent isconverted into a voltage, so that the integrator 20 outputs the voltage.Note that, after the cell select process is terminated, the constantcurrent source connect switch 41 is turned OFF and the output of theintegrator 20 is read. After the reading of the output of the integrator20 is performed, the constant current source connect switch 41 is turnedON again. The signal output from the integrator 20 is converted into adigital signal by an AD converter (which will be described later) andthe photoelectric conversion device 2 outputs the digital signal. Notethat the constant current source 40 is connected to the output line 30,and the photocurrent flows into the integrator 20 after the currenti_(B) set up by the constant current source 40 is reduced from thephotocurrent output from the phototransistor 11.

Subsequently, the photoelectric conversion device 2 performs a cellreset process to discharge the signal charge accumulated in thephototransistor 11. Specifically, the cell reset switch 13 is turned ONand the integrator connect switch 220 is turned OFF (step S24). At thistime, in the photoelectric conversion device 2, as illustrated in FIG.15, the cell select switch 12 is in an ON state, the cell reset switch13 is in an ON state, the integrator reset switch 24 is in an OFF state,and the integrator connect switch 220 is in an OFF state. Thereby, afeedback loop is formed by the base region B and the emitter region E ofthe phototransistor 11, the cell select switch 12, the output line 30,the feedback amplifier 210, the reset line 50, and the cell reset switch13. An electric potential of the output line 30 is controlled to beequal to the reference voltage V_(ref), and the base-to-emitter voltageV_(BE) of the phototransistor 11 is controlled to be equal to a valuethat allows the passage of the current i_(B) set up by the constantcurrent source 40 even if the phototransistor 11 has differentcharacteristics.

Subsequently, after the process of step S24 is completed, the operationof the photoelectric conversion device 2 is returned to step S21 again,and the above-described processes of steps S21 to S24 are repeated.

According to the above-described photoelectric conversion device 2 ofthe second embodiment, in addition to the advantageous effects of thefirst embodiment, the following advantageous effect is obtained. Namely,the photoelectric conversion device 2 of the second embodiment includesthe feedback amplifier 210 for use when performing the cell resetprocess and the feedback amplifier 210 is separate from the operationalamplifier 21 of the integrator 20. Hence, it is possible for the secondembodiment to provide a flexible design for the photoelectric conversiondevice 2. There may be a case in which it is demanded that theoperational amplifier 210 and the operational amplifier 21 have mutuallydifferent characteristics. In such a case, according to thephotoelectric conversion device 2 of the second embodiment, the optimaloperational amplifiers may be selected for both the requiredcharacteristics.

In the second embodiment, a case in which the integrator reset switch 24is in an OFF state during the cell reset process has been described.However, it is to be understood that the second embodiment describedabove is exemplary and explanatory and is not restrictive of theinvention as claimed. For example, the integrator reset switch 24 may bein an ON state during the cell reset process.

Third Embodiment

A configuration of a photoelectric conversion device 3 according to athird embodiment will be described. FIG. 16 is an equivalent circuitdiagram illustrating the photoelectric conversion device 3 according tothe third embodiment.

As illustrated in FIG. 16, the photoelectric conversion device 3according to the third embodiment is essentially the same as thephotoelectric conversion device 1 according to the first embodiment,except that the photoelectric conversion device 3 includes two constantcurrent sources (a first constant current source 311 and a secondconstant current source 312). Other elements of the photoelectricconversion device 3 of the third embodiment are essentially the same ascorresponding elements of the photoelectric conversion device 1 of thefirst embodiment, and a description thereof will be omitted.

As illustrated in FIG. 16, the photoelectric conversion device 3includes the pixel cell 10, the integrator 20, the output line 30, thereset line 50, the first constant current source 311, the secondconstant current source 312, a first constant current source connectswitch 321, and a second constant current source connect switch 322.

The pixel cell 10 is an example of the photoelectric conversion unit,and the above-described configuration including the integrator 20, theoutput line 30, the reset line 50, the second constant current source312, and the second constant current source connect switch 322 is anexample of the base potential setting unit. Further, the above-describedconfiguration including the first constant current source connect switch321 and the second constant current source connect switch 322 is anexample of a switch unit.

The first constant current source 311 may be a constant current sourcedisposed to branch off from the path of the output line 30 and connectedto the output line 30 via the first constant current source connectswitch 321. This first constant current source 311 is used to performthe cell select process, and when the first constant current sourceconnect switch 321 is turned ON, the first constant current source 311is connected to the output line 30.

The second constant current source 312 may be a constant current sourcedisposed to branch off from the path of the output line 30 and connectedto the output line 30 via the second constant current source connectswitch 322. This second constant current source 312 is used to performthe cell reset process, and when the second constant current sourceconnect switch 322 is turned ON, the second constant current source 312is connected to the output line 30.

Next, an example of operation of the photoelectric conversion device 3according to the third embodiment will be described. FIG. 17 is aflowchart for explaining operation of the photoelectric conversiondevice 3 according to the third embodiment.

Upon reception of instructions from a control unit (not illustrated) viathe control lines 61 and 62, the photoelectric conversion device 3performs various processes of steps S31 to S34 as illustrated in FIG.17. FIGS. 18 to 21 are diagrams illustrating various states of thephotoelectric conversion device 3 according to the third embodiment whenperforming the various processes.

First, the photoelectric conversion device 3 performs an accumulationprocess to accumulate signal charge in the phototransistor 11.Specifically, the cell select switch 12 is turned OFF and the cell resetswitch 13 is turned OFF (step S31). At this time, in the photoelectricconversion device 3, as illustrated in FIG. 18, the cell select switch12 is in an OFF state, and the cell reset switch 13 is in an OFF state.Note that the integrator reset switch 24, the integration capacitorconnect switch 23, the first constant current source connect switch 321,and the second constant current source connect switch 322 may be in anON state or in an OFF state. In the phototransistor 11, upon receptionof an incident optical energy, the optical energy is converted intosignal charge and the signal charge is accumulated in the base region B.In this condition, the emitter region E and the base region B of thephototransistor 11 of the pixel cell 10 are in a floating state. Hence,the signal charge generated according to the optical energy isaccumulated in the base region B starting from the base potential set upby the cell reset process (which will be described later), and thebase-to-collector voltage V_(BC) is reduced.

Subsequently, the photoelectric conversion device 3 performs anintegrator reset process to discharge the signal charge in the capacitor22 of the integrator 20. Specifically, the integrator reset switch 24 isturned ON and the integration capacitor connect switch 23 is turned ON(step S32). At this time, in the photoelectric conversion device 3, asillustrated in FIG. 19, the cell select switch 12 is in an OFF state,the cell reset switch 13 is in an OFF state, the integration capacitorconnect switch 23 is in an ON state, and the integrator reset switch 24is in an ON state. Thereby, the signal charge in the capacitor 22 may bedischarged. Note that the first constant current source connect switch321 and the second constant current source connect switch 322 at thistime may be in an ON state or in an OFF state. It is preferable thatthese switches are in an OFF state, in order to reduce the error due tothe ON resistance when the current flows through the first constantcurrent source connect switch 321 and the second constant current sourceconnect switch 322.

Subsequently, the photoelectric conversion device 3 performs a cellselect process to discharge the signal charge accumulated in thephototransistor 11. Specifically, the cell select switch 12 is turnedON, the integrator reset switch 24 is turned OFF, the first constantcurrent source connect switch 321 is turned ON, and the second constantcurrent source connect switch 322 is turned OFF (step S33). At thistime, in the photoelectric conversion device 3, as illustrated in FIG.20, the cell select switch 12 is in an ON state, the cell reset switch13 is in an OFF state, the integration capacitor connect switch 23 is inan ON state, the integrator reset switch 24 is in an OFF state, thefirst constant current source connect switch 321 is in an ON state, andthe second constant current source connect switch 322 is in an OFFstate. Hence, the photocurrent according to the signal chargeaccumulated in the phototransistor 11 flows into the integrator 20. Inthe integrator 20, the photocurrent generated in the reading time isaccumulated and the photocurrent is converted into a voltage, so thatthe integrator 20 outputs the voltage. Note that, after the cell selectprocess is terminated, the constant current source connect switch 41 isturned OFF and the output of the integrator 20 is read. After thereading of the output of the integrator 20 is performed, the constantcurrent source connect switch 41 is turned ON again. The signal outputfrom the integrator 20 is converted into a digital signal by an ADconverter (which will be described later) and the photoelectricconversion device 3 outputs the digital signal. Note that the firstconstant current source 311 is connected to the output line 30, and thephotocurrent flows into the integrator 20 after a constant currenti_(B1) set up by the first constant current source 311 is reduced fromthe photocurrent output from the phototransistor 11.

Subsequently, the photoelectric conversion device 3 performs a cellreset process to discharge the signal charge accumulated in thephototransistor 11. Specifically, the cell reset switch 13 is turned ON,the integration capacitor connect switch 23 is turned OFF, the firstconstant current source connect switch 321 is turned OFF, and the secondconstant current source connect switch 322 is turned ON (step S34). Atthis time, in the photoelectric conversion device 3, as illustrated inFIG. 21, the cell select switch 12 is in an ON state, the cell resetswitch 13 is in an ON state, the integration capacitor connect switch 23is in an OFF state, the integrator reset switch 24 is in an OFF state,the first constant current source connect switch 321 is in an OFF state,and the second constant current source connect switch 322 is in an ONstate. Thereby, a feedback loop is formed by the base region B and theemitter region E of the phototransistor 11, the cell select switch 12,the output line 30, the operational amplifier 21 of the integrator 20,the reset line 50, and the cell reset switch 13. An electric potentialof the output line 30 is controlled to be equal to the reference voltageV_(ref), and the base-to-emitter voltage V_(BE) of the phototransistor11 is controlled to be equal to a value that allows the passage of aconstant current i_(B2) set up by the second constant current source 312even if the phototransistor 11 has different characteristics.

Subsequently, after the process of step S34 is completed, the operationof the photoelectric conversion device 3 is returned to step S31 again,and the above-described processes of steps S31 to S34 are repeated.

According to the above-described photoelectric conversion device 3 ofthe third embodiment, in addition to the advantageous effects of thefirst embodiment, the following advantageous effect is obtained. Namely,the photoelectric conversion device 3 of the third embodiment includesthe two constant current sources (the first constant current source 311and the second constant current source 312). Hence, the current pulledwhen performing the cell select process, and the current pulled whenperforming the cell reset process may be set up arbitrarily.Specifically, the current pulled when performing the cell reset processis used to set up the initial bias state of the phototransistor 11, andthe current affects the time to stabilize the initial bias state and thetime to output the photocurrent. Further, the current pulled whenperforming the cell select process affects the dynamic range of theoutput of the integrator 20 and the impedance of the output line 30.Hence, the current value required for the constant current source 40 isnot necessarily the same, and there may be a case in which it ispreferable that the current value required when performing the cellselect process is different from that when performing the cell resetprocess. In such a case, according to the photoelectric conversiondevice 3 of the third embodiment, the optimal constant current source 40may be selected for the respective purposes.

In the third embodiment, a case in which the photoelectric conversiondevice 3 includes the two constant current sources (the first constantcurrent source 311 and the second constant current source 312) has beendescribed. However, it is to be understood that the third embodimentdescribed above is exemplary and explanatory and is not restrictive ofthe invention as claimed. For example, the photoelectric conversiondevice 3 may include three or more constant current sources.

Fourth Embodiment

In a fourth embodiment, an image generation device in which a pluralityof photoelectric conversion devices, each configured as described above,are arrayed in a two-dimensional formation will be described. FIG. 22 isa diagram illustrating a configuration of the image generation deviceaccording to the fourth embodiment.

As illustrated in FIG. 22, the image generation device includes atwo-dimensional array of pixel cells 10-1-1 to 10-M-N, a plurality ofrow selection lines 410-1 to 410-M, a plurality of column output lines420-1 to 420-N, a plurality of reset lines 430-1 to 430-N, a rowselector 440, an IV (current-to-voltage) conversion array 450 ofintegrators 20, and an AD (analog-to-digital) conversion array 460 of ADconverters to convert the signals output from the integrators 20 intodigital signals.

Each of the pixel cells 10-1 to 10-M is connected to a corresponding oneof the row selection lines 410 and further connected to a correspondingone of the column output lines 420. The pixel cells 10-1-n to 10-M-nconnected to the row selection lines 410-1 to 410-M, respectively, areconnected to a column output line 420-n (1≦n≦N) of the column outputlines 420-1 to 420-N.

The row selector 440 selectively activates one pixel cell 10-m-n (1≦m≦M)of the pixel cells 10-1-n to 10-M-n by using the row selection lines410-1 to 410-M. The active pixel cell 10-m-n transfers, upon receptionof an incident optical energy, the photocurrent having the magnitudeaccording to the intensity of the incident light to the IV conversionarray 450 via a corresponding column output line 420-n. Further, a resetpotential is given to the active pixel cell 10-m-n via a correspondingreset line 430-n.

The IV conversion array 450 converts the output current from each of thepixel cells 10 into the output voltage.

The AD conversion array 460 performs the analog-to-digital conversion ofthe output voltages of the IV conversion array 450 to generate an outputimage signal.

Note that a pair of one IV conversion array 450 and one AD conversionarray 460 may be arranged for every one of the columns of thetwo-dimensional array, or may be arranged for every two or more columnsof the two-dimensional array. In these cases, the column selectionfunction may be added to the pair of the arrays 450 and 460.

As described above, the image generation device of the fourth embodimentis configured so that a plurality of photoelectric conversion deviceseach according to any of the first through third embodiments are arrayedin a two-dimensional formation. Hence, the reading time may be reduced,and therefore the image generation speed may be increased.

According to the above-described embodiments, it is possible to providea photoelectric conversion device which is able to reduce the timeneeded to reset the electric charge accumulated in the phototransistor.

The photoelectric conversion device according to the present disclosureis not limited to the above-described embodiments, and variations andmodifications may be made without departing from the scope of thepresent disclosure.

What is claimed is:
 1. A photoelectric conversion device comprising: aphotoelectric conversion unit including a phototransistor having acollector region, an emitter region, and a base region to generate anoutput current according to an intensity of incident light to thephototransistor; and a base potential setting unit configured to set upa base potential of the phototransistor so that the output current fromthe photoelectric conversion unit is equal to a predetermined currentvalue.
 2. The photoelectric conversion device according to claim 1,wherein: the base potential setting unit is connected to the base regionof the phototransistor and includes a reset line to reset thephototransistor; and the photoelectric conversion unit comprises aswitch configured to connect the base region to the reset line anddisconnect the base region from the reset line.
 3. The photoelectricconversion device according to claim 2, wherein: the base potentialsetting unit comprises an integrator configured to perform anintegration process of the current output from the photoelectricconversion unit, the integration process accumulating electric chargeaccording to the current within a predetermined time, converting theaccumulated charge into a voltage, and outputting the voltage; and thereset line is connected to an output of the integrator.
 4. Thephotoelectric conversion device according to claim 2, wherein: the basepotential setting unit comprises an operational amplifier configured toreceive the current output from the photoelectric conversion unit; andthe reset line is connected to an output of the operational amplifier.5. The photoelectric conversion device according to claim 1, wherein thebase potential setting unit comprises a constant current sourceconnected to the emitter region of the phototransistor.
 6. Thephotoelectric conversion device according to claim 1, wherein the basepotential setting unit comprises: two or more constant current sourceseach connectable to the emitter region of the phototransistor; and aswitch unit configured to be switched between a first connection statein which one of the constant current sources is connected to the emitterregion to enable the base potential setting unit to set up the basepotential and a second connection state in which another of the constantcurrent sources is connected to the emitter region to disable the basepotential setting unit from setting up the base potential.
 7. Thephotoelectric conversion device according to claim 1, wherein aplurality of photoelectric conversion units, each including thephototransistor, are arrayed in a two-dimensional formation.
 8. An imagegeneration device comprising: a photoelectric conversion device; and anAD converter configured to convert an output current from thephotoelectric conversion device into a digital signal, the photoelectricconversion device including: a photoelectric conversion unit including aphototransistor having a collector region, an emitter region, and a baseregion to generate the output current according to an intensity ofincident light to the phototransistor; and a base potential setting unitconfigured to set up a base potential of the phototransistor so that theoutput current from the photoelectric conversion unit is equal to apredetermined current value.
 9. The image generation device according toclaim 8, wherein: the base potential setting unit is connected to thebase region of the phototransistor and includes a reset line to resetthe phototransistor; and the photoelectric conversion unit comprises aswitch configured to connect the base region to the reset line anddisconnect the base region from the reset line.
 10. The image generationdevice according to claim 9, wherein: the base potential setting unitcomprises an integrator configured to perform an integration process ofthe current output from the photoelectric conversion unit, theintegration process accumulating electric charge according to thecurrent within a predetermined time, converting the accumulated chargeinto a voltage, and outputting the voltage; and the reset line isconnected to an output of the integrator.
 11. The image generationdevice according to claim 9, wherein: the base potential setting unitcomprises an operational amplifier configured to receive the currentoutput from the photoelectric conversion unit; and the reset line isconnected to an output of the operational amplifier.
 12. The imagegeneration device according to claim 8, wherein the base potentialsetting unit comprises a constant current source connected to theemitter region of the phototransistor.
 13. The image generation deviceaccording to claim 8, wherein the base potential setting unit comprises:two or more constant current sources each connectable to the emitterregion of the phototransistor; and a switch unit configured to beswitched between a first connection state in which one of the constantcurrent sources is connected to the emitter region to enable the basepotential setting unit to set up the base potential and a secondconnection state in which another of the constant current sources isconnected to the emitter region to disable the base potential settingunit from setting up the base potential.
 14. The image generation deviceaccording to claim 8, wherein a plurality of photoelectric conversionunits, each including the phototransistor, are arrayed in atwo-dimensional formation.